The invention relates to a bistable optical device using a photorefractive crystal and based on the index variations induced in this photorefractive crystal. The invention also applies to a bistable matrix for providing, in parallel logic functions between optical beams.
An optical bistable element is characterized by the existence of two stable states of the transmitted intensity corresponding to the same value of the incident intensity. An optical bistable element is generally formed by a non linear medium placed inside a Fabry-Perot resonating cavity. By varying the intensity of the incident signal beam, the index of the non linear medium is modified and consequently the resonating cavity is detuned. Under these conditions, two stable values of the transmitted (or reflected) intensity can be obtained as well as a non linear characteristic of the transmitted intensity as a function of the incident intensity. Thus a function is obtained similar to that of an electronic transistor and which may be termed optical transistor. The description of such a device can be found in the article "Premiers pas du transistor optique" by Patricia PINEAU published in the review "La Recherche" n.degree. 114 of September 1980.
FIG. 1 shows such a device in a simplified way and includes a non linear medium enclosed by reflection devices R1 and R2. This device receives a wave of intensity Ii and transmits a wave of intensity It.
Such devices have already been shown in the laboratory using the following non linear media:
Kerr optical effect in liquids, PA1 excitonic resonance in super network semiconductors, PA1 saturated absorption in gases. PA1 a photorefractive crystal with a first and a second face; PA1 a first mirror applied to the first face; PA1 a second mirror applied to the second face; PA1 a first light source which emits a light beam illuminating the first face uniformly with a given intensity through the first mirror; PA1 at least a second light source which emits a control signal beam of given intensity, also illuminating the first face through the first mirror and focused on a given portion of the photorefractive crystal so as to cause bistable switching of this photorefractive crystal portion.
It is now well known that the operating points of a device are obtained by intersection of the resonance curve of the Fabry-Perot cavity with a straight line turning about a fixed point and whose slope depends on the incident intensity (FIG. 2). Depending on the position of the operating point, a hysteresis cycle or a non linear characteristic may be obtained between the transmitted intensity and the incident intensity.
The invention relates to a type of bistable element based on the index variations induced by photorefractive effect in electro-optical crystals.